Perianth
The outer, or accessory, organs in the flower i.e., the sepals and petals, which protect the essential organs of reproduction, or the true flower, in botanical language, i.e., the stamens and pistil. In most flowers, the perianth is double, consisting of an outer circle of green sepals, and an inner circle of coloured petals (as in Abutilon, Fig. 83). In the Liliaceae, and other plants, the perianth, though double, consists of parts very much alike, and all petaloid (Lilium concolor, Fig. 84); while on others--e.g., Juncus, &c.--all the parts are sepaloid. Often one of the circles is wanting, and the missing circle is usually the petals, though (as in Anemone, Fig. 85) those present may appear petaloid. Often such single perianths are sepaloid (Mercurialis). In a good many plants (Willows, Arum), there is no trace of perianth. In many plants, the bracts are modified to simulate part of a perianth (sepaloid bracts of Dianthus, and petaloid bracts of Cornus suecica). PERIDERMIUM from peri, around, and derma, skin). A group of Fungi, including only a few forms, parasitic on coniferae. Those of most importance are P. elatinum (also called AEcidium elatinum), which gives rise to swellings on the branches of the Silver Fir (Abies pectinata), and P. Pini, with its variety P. acicolum, on the Scotch Fir (Pinus sylvestris) and the Austrian Fir (P. austriaca), in pleasure grounds, as well as in forests. The genus belongs to the group of "red rusts," and the Fungi in it from a mycelium in the tissues of the leaves and branches of the host-plants. On the surface of the diseased parts there appear, after a time, cylindrical, laterally flattened, or conical outgrowths, called peridia, 1/10 in. To 1/8 in. high, 1/20 in. to 1/4 in. broad; the larger forms being on the branches, and the smaller on the needles. These outgrowths are formed of a thin layer of cells, which tears irregularly at the tip, and sets free numerous small, round, orange-coloured spores, to reproduce the plant.
P. (AEcidium) elatinum is not very frequent; but where it is present the tree sometimes bears a large number of swellings on the branches, often near the base of one or more small branches, or supports what are known as "witch-besoms" on the larger branches. Both kinds of growth are the work of the Fungus; and the difference is due chiefly to age. The swellings vary from the size of a Hazel nut to that of a closed fist; the bark is the part most enlarged. The mycelium grows in the bark and the pith-rays and pith, and often causes separation of the bark from the wood, and the death of the wood in patches. In most cases, it does not kill the branch entirely; but the mycelium is perennial, and the growth on the branch continues to increase year after year. When branches grow from the surface, they often branch very freely, but remain stunted; and this is repeated year by year, the "witch-besoms" being thus formed, and are usually covered with peridia. Though not usually, or only very slowly, dangerous to the life of the tree, the growths are very objectionable, as they greatly spoil its appearance. The only effectual remedy is cutting off the diseased branches as soon as the swellings appear, in order to prevent the disease from spreading to other trees. P. Pini grows in branches of Firs, and almost always on trees under twenty years old. The peridia are about 1/5 in. or 1/4 in. long, and 1/8 in. high, and are usually flattened on the sides. They are generally crowded (see Figs. 86 and 87), and may occur on the same branch for two or more years in succession; but they ultimately kill the branches. The Fungus is also found on the needles, in a form called P. acicolum. The peridia are much smaller in this form, and are more cylindrical, and stand in a row along the needles; but the latter are not so severely injured as the branches. The growth of the mycelium in the branches causes the conversion of the starch and other food stores in the cells into turpentine and resins, which soak into all the tissues, and at last often appear on the surface of the branches. investigations by Wolff and others have, of late years, led to the belief that this Fungus is only a stage in the life-cycle of another, named Coleosporium Senecionis, which has long been known as a widespread and abundant parasite on the leaves and stems of Groundsels (Senecio vulgaris and S. viscosus) and of Ragwort (S. Jacobaea). The Coleosporium forms yellow or orange spots, crowded into irregular patches, or in concentric rings, or scattered over the leaves. These spots are composed of masses of small, orange-yellow spores, covered with fine warts, and nearly round, or cylindrical or clavate in form. Experiments in growing the Fir-parasite from these spores so often fail, that further evidence on their connection is desirable. Almost the only available remedy is to cut off and burn all parts of the trees that show the disease, and to keep the neighbourhood of the trees as clear as possible of species of Senecio.